The contributions of modern physics have increased the availability of radioactive X-ray and gamma-ray emitting materials in industry and nuclear medicine. As radioactive emission principably occurs outside of the visible part of the electromagnetic spectrum, an unaided human observer is unable to "see" a source of radioactive emission. It is difficult, therefore, to distinguish a source of X-ray and gamma-ray emission from non-emitting neighboring and visually similar objects. Various techniques exist to locate a source of radioactive emission. One technique requires trial and error search with a Geiger counter. Another technique uses a scintillation detector. The information provided by these techniques is limited to the intensity and general location of radioactive emission, and reveals nothing about the shape of the radioactive object, or the distribution of radioactivity within the object. An X-ray camera formed by placing X-ray sensitive film behind a pin hole in an X-ray shield provides a recording of a two-dimensional facsimile of an X-ray or gamma-ray emitting object in one perspective. The facsimile can be viewed after a delay for processing of the film. Furthermore, a single pin hole aperture camera is rendered extremely inefficient by the miniature aperture of the pin hole.
Other, existing X-ray or gamma-ray cameras employ either parallel, converging or diverging collimators to bring an essentially aligned beam projection of a radioactive object onto a detector. The detector may be in the nature of a film, a scintillator, or a phosphor material which converts X-rays and gamma-rays into visible light, or a combination of a scintillator and phosphor. The visible light generated, together with positional information, is then processed by any of a wide variety of methods using such devices as photo-multiplier tubes (e.g., Anger cameras), image intensifiers, visible light cameras, video cameras, and centroid-computing electronics in various combinations. Without the additional steps of making successive exposures and subsequent reconstructions, a particular object-to-camera geometry provides a two-dimensional single perspective image of an X-ray or gamma-ray emitting object.
Although versatile generally, the Anger camera provides course spatial resolution, and is physically large and expensive, all characteristics which make it disadvantageous in certain specific application areas. With complex centroiding electronic circuitry, an Anger camera is capable of providing images with spatial resolution on the order of 2 to 3 millimeters. An Anger camera enhanced by centroiding electronics must iteratively calculate the energy and position of each impinging gamma-ray, thereby necessarily incurring time delays which prevent the camera from processing images resulting from high rates of gamma-ray impingement with more than one hundred thousand events per second. The Anger camera suffers from dead-time during which its circuitry is unable to process and consequently loses, data about the images. Consequently, the performance and resolution of Anger cameras are limited substantially by their ancillary electronic circuitry.
Presently available Anger cameras have rather poor sensitivity for radioisotopes which emit either gamma-rays or X-rays with energies predominantly below 80 keV (kiloelectron volts) such as iodine-125, xenon-133 and thalium-201. Moreover, in dynamic studies of angiocardiography using the ultra short-lived radioisotopes such as irridium-191m with a half-life of 4.9 seconds, the gamma camera must be able to handle exceptionally high counting rates as well as provide high spatial resolution. In such cases energy information and large imaging area are not essential. The dead-times of a conventional Anger camera, is too long and the resolution too course for such applications.
An earlier invention, a low intensity X-ray image scope "Lixiscope" disclosed in U.S. Pat. No. 4,142,101 is a fully portable device which provides an intensified visible-light image of objects illuminated with point X-ray or gamma-ray sources (e.g. iodine 125 of between approximately 50 milliCuries or to 100 milliCuries or an X-ray generator) in real time. It uses an X-ray to visible-light convertor to drive a visible-light image intensifier having one or more microchannel plate electron multipliers. The Lixiscope provides a viewer with a visible shadow, in real time, of the X-ray or gamma-ray illuminated objects.
The Lixiscope requires either an X-ray generator or a radioactive source such as iodine 125, cadmium 109 or tin 119m to illuminate the object being viewed and thereby provide a shadow of that object upon a scintillator or rare-earth phosphor. The source, if radioactive, must be periodically replaced at times determined by the radioactive half-life of the source.
Applications for the Lixiscope are limited principally to the field of fluoroscopy or intensifier-assisted radiography by a need to insert the object (e.g. a human tooth or part of a human hand) between the source and the converter. Moreover, in practice, unless the Lixiscope views an object illuminated with a point X-ray or gamma-ray source, it is unable to provide a sharp image of the object's shadow. Consequently, the Lixiscope is unable to view extended sources of x-ray and gamma-ray emitting objects and therefore, is unsuitable for applications in the field of nuclear medicine where a low dosage of a radioactive isotope is introduced into the human body and reacts with particular anatomical organs to provide an extended object emitting X-rays and gamma-rays. Similarly, there is currently a need in industry for a device capable of forming visible light images of extended objects emitting X-rays, gamma-rays, and charged or uncharged particles, as well as a portable, hand-held device for scanning objects and detecting, and providing visible light images, of the sites of areas emitting radioactive emission.